1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read and/or write terminal. Generally, transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read and write terminal.
2. Discussion of the Related Art
FIG. 1 very schematically shows a conventional example of a data exchange system of the type to which the present invention relates between a read/write terminal 1 and a transponder 10.
Generally, terminal 1 is essentially formed of a series oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1, between an output terminal 2 of an amplifier or antenna coupler 3 and a terminal 4 at a reference potential (generally, the ground). Amplifier 3 receives a high-frequency transmission signal E, provided by a modulator 5 (MOD1), which receives a reference frequency (signal OSC), for example, from a quartz oscillator (not shown). Modulator 5 receives, if necessary, a data signal Tx to be transmitted and, in the absence of a data transmission from the terminal, provides the high-frequency carrier (for example, at 13.56 MHz) adapted to remotely supply a transponder. In receive mode, terminal 1 uses a demodulator 6 (DEMOD1), which is used to detect a load variation generated by transponder 10 on the high-frequency signal. Demodulator 6 samples, for example, the voltage across terminals 7 and 4 of capacitor C1, and provides a signal Rx of data received after demodulation.
Other circuits, not shown, generally complete a terminal 1. Among these circuits, a circuit for controlling and exploiting the received data most often based on a microprocessor for processing the control signals and the data, may be included, among others. These circuits generally communicate with different input/output circuits (keyboard, screen, means of transmission to a server, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal draw the power required by their operation from a supply circuit (not shown) connected, for example, to the electric supply system or to batteries.
A transponder 10, intended for cooperating with a terminal 1, essentially includes a parallel oscillating circuit formed of an inductance L2, in parallel with a capacitor C2 between two input terminals 11, 12 of a control and processing circuit 13. Terminals 11, 12 are in practice connected to the input of a rectifying means (not shown), outputs of which form D.C. supply terminals of the circuits internal to the transponder. These circuits generally include, essentially, a microprocessor 14 (P) capable of communicating with other elements (for example, a memory) through connections 15. Transponder 10 further includes a demodulator 16 (DEMOD2) of the signals received from terminal 1, which provides a signal Rx′ to circuit 14, and a modulator 17 (MOD2) for transmitting to the terminal data Tx′ that it receives from circuit 14.
The oscillating circuits of the terminal and of the transponder are generally tuned on a same frequency corresponding to the frequency of an excitation signal of the terminal's oscillating circuit. This high-frequency signal (for example, at 13.56 MHz) is not only used as a transmission carrier but also as a remote supply carrier for the transponder(s) located in the terminal's field. When a transponder 10 is located in the field of a terminal 1, a high-frequency voltage is generated across terminals 11 and 12 of its resonant circuit. This voltage, after being rectified and possibly clipped, is intended for providing the supply voltage of electronic circuits 13 of the transponder. For clarity, the rectifying, clipping, and supply means have not been shown in FIG. 1. It should be noted that, generally, the demodulation (block 16) is performed upstream of the clipping means to keep the amplitude modulation of the data on the high-frequency carrier transmitted by the terminal. This amplitude modulation is performed according to different coding techniques to transmit data and/or control signals to the transponders. In return, data transmission Tx′ from the transponder to a terminal is generally performed by modulating the load formed by resonant circuit L2, C2. This is why modulator 17 has been shown in parallel with this resonant circuit. The load variation is performed at the rate of a so-called back-modulation sub-carrier, of a frequency (for example, 847.5 kHz) smaller than that of the carrier.
The load variation coming from a transponder can then be detected by the terminal in the form of an amplitude variation or of a phase variation by means, for example, of a measurement of the voltage across capacitor C1 or of the current in the oscillating circuit by means of demodulator 6.
The present invention more specifically applies to systems having a read and/or write terminal using an amplitude demodulation to detect the load variation of a transponder in its field and thus demodulate the transmitted data.
A problem that is posed in conventional electromagnetic transponder systems is that a transponder remotely supplied by a terminal and transmitting data to said terminal may be undetected by the terminal, that is, the terminal's demodulator cannot manage to detect the presence of a data modulation. This phenomenon is generally called a “demodulation gap”. For a given system, this corresponds to a relative position of a terminal and of a transponder to which the terminal's demodulator is “blind”.
It should be noted that this notion of a demodulation gap is different from what is called a “remote supply gap” where the transponder cannot manage to be supplied by the high-frequency signal, even though it is in the terminal's electromagnetic field. Indeed, there exists a relative position between a transponder and a terminal at which the magnetic coupling between oscillating circuits is such that the transponder is not supplied, that is, the voltage recovered across terminals 11 and 12 of its oscillating circuit is too small for it to operate. In a demodulation gap, the transponder is properly supplied. It generally properly detects the data transmitted by the terminal in amplitude modulation. It properly transmits data to the terminal in back-modulation, by variation of the load of its oscillating circuit. However, the terminal's demodulator does not detect this back-modulation.
As a result of this demodulation gap problem, a terminal cannot detect a transponder present in its field since this detection conventionally uses the result of the data demodulator on the terminal side. In particular, when it is in a stand-by state, waiting for a transmission, the terminal periodically transmits interrogation requests by modulating the amplitude of the remote supply carrier. The terminal then monitors the output of its demodulator, which will indicate thereto the presence of a transponder. Indeed, where a transponder is “woken up” by its entering the field of a terminal, it demodulates the interrogation message periodically transmitted by this terminal and answers it to have itself identified.
An additional disadvantage is that, since the transponder has received data from the terminal, it believes that it is identified by the terminal, which is not true. The only present techniques to isolate this phenomenon are to multiply the information exchanges to validate the transmission, which is costly in terms of transmission duration.
Different transponder systems of the type to which the present invention applies are described, for example, in U.S. Pat. Nos. 4,963,887 and 5,550,536, as well as in European patent applications no. 0,722,094 and 0,857,981, all of which are incorporated herein by reference.
In a read/write terminal provided with an amplitude demodulator, the output voltage of a demodulator annuls, that is, there is a demodulation gap, in two frequency configurations of the carrier (13.56 MHz) which, for a given coupling coefficient between the oscillating circuits of the terminal and of the involved transponder, surround the self-resonant frequency of oscillating circuit L2–C2 of the transponder. Ideally, the median frequency corresponds to the perfect tuning of the terminal and of the transponder on the remote supply carrier frequency, where the amplitude available for the demodulation is maximum.
It is generally desired to have both the oscillating circuits of the terminal and of the transponder tuned on the remote supply carrier frequency, to maximize the remote supply power received by the transponder. However, the manufacturing tolerances of the capacitors used for the oscillating circuits, especially for capacitor C2 of the transponder, which is generally integrated, generally are on the order of 10%. As a result of the extent of these manufacturing tolerances, perfect tuning is practically not respected and it cannot be guaranteed that a transponder entering the field of a terminal will not be, in a given coupling position, in a demodulation gap.
Further, the position of demodulation gaps in the amplitude demodulator response varies according to the mutual inductance between the oscillating circuits. Now, this mutual inductance depends on the distance separating antennas L1 and L2 of the terminal and of the transponder, and thus on the relative position of the transponder with respect to the terminal upon transmission.
The combined problems of the existence of demodulation gaps and of the variation of the position of these demodulation gaps with respect to the distance between the inductances, associated with the manufacturing tolerances of the components, make conventional systems rather unreliable.